Prior to the present invention, circuit boards were often made by etching copper-clad laminates in accordance with standard negative photo-resist procedures. In particular situations, where cost was not a problem and where long, air polluting cures could be tolerated, conductive .Iadd.pastes or .Iaddend.inks were available which could be directly applied onto the substrate surface in a patterned manner.
Although conductive pastes often simplify the production of circuit boards in many instances, the conductive pastes are frequently limited to particular substrates depending upon the manner by which the paste is cured. The cure of one available paste, for example, is based on the evaporation of a volatile solvent. However, organic solvents often damage thermoplastic substrates when allowed to contact substrate surfaces over extended evaporation periods. In addition, air pollution hazards .[.has.]. .Iadd.have .Iaddend.reduced the use of such materials for mass production applications.
Another type of curable ink must be fired at an elevated temperature before it becomes conductive. A conductive powder is used in combination with a glass, as shown for example by J. E. Jolley, Solid State Technology, May 1974, pages 33-37. In order to render the ink conductive, it is necessary to heat the ink at .[.temperature.]. temperatures up to 900.degree. C. As a result, such conductive inks can only be employed on ceramic substrates which are able to withstand such extreme temperature conditions.
As shown by Ehrreich et al. U.S. Pat. No. 3,202,488 and .[.Gillard.]. .Iadd.Gilliland .Iaddend.U.S. Pat. No. 3,412,043, additional curable inks are available based on the employment of an epoxy resin which can be employed in the form of either a .[.one-package.]. .Iadd.one package .Iaddend.or .[.two-package.]. .Iadd.two package .Iaddend.system. An advantage of the two package system is that a low temperature cure can be achieved. However, the catalyst must be mixed with the resin immediately prior to use, and the ink remains tacky for several hours. In addition, the two package method cannot be used to make circuit boards in a continuous manner, since a batch of mixture having a relatively short pot life must be freshly prepared. Although the .[.one-package.]. .Iadd.one package .Iaddend.system allows for faster cures, higher temperatures are required to release the curing catalyst. Accordingly, such epoxy systems are either undesirable on thermoplastic substrates, which cannot tolerate elevated cure temperature, or are unsuitable for economic mass production procedures because of long cure times.
Although recent improvements have eliminated, in many instances, the need for time consuming etching procedures for making circuit boards, based on the direct application of curable inks which can be rendered conductive upon cure, no curable printing inks are available which can be rapidly cured within two minutes or less to the conductive state and which can be used on a wide variety of substrates, including thermoplastic substrates.
.Iadd.When certain organic resins are exposed to radiation such as ultraviolet radiation, the resin can be cured into a non-tacky but relatively flexible and well-adhering coating on a thermoplastic substrate in a short period of time and without subjecting the thermoplastic substrate to high temperature conditions which would damage the substrate. While a number of such resins are known, they cannot be effectively cured by ultraviolet radiation if the most conventionally used electrically conductive fillers are present in the resins in such quantity as to impart sufficient electrical conductivity to the coating. Conductive metals shield the transmission of ultraviolet radiation through the matrix and, if the filler is primarily in the form of fine metallic particles or flakes, the radiation is blocked to an extent that the resin does not cure except for a thin surface layer thereof.
If the filler is in the form of large, coarse particles, the interstitial spaces between the particles may enable the transmission of sufficient radiation to adequately cure the resin. Coarse particles, however, do not contact one another with such intimacy as to establish good electrical conduction through the coating.
The present invention contemplates the provision of a coating which may be quickly and adequately cured on a thermoplastic substrate by ultraviolet radiation, which possesses good electrical conductivity and which, at the same time, maintains good adhesion and flexibility after being cured. This is achieved by providing a coating formed by an ultraviolet-cured organic resin matrix and by at least about 40% and not more than about 90% by weight of electrically conductive particles, at least about 85% by weight of the particles being substantially spherical and having a diameter of between 10 and 50 microns. Because a substantial majority of the conductive particles are spherical and are within the stated diameter range, they are substantially regular in shape and are sufficiently small as to be capable of establishing good electrically conductive contact with one another. At the same time, the particles are sufficiently large to leave interstitial spaces which are capable of enabling the transmission of sufficient ultraviolet radiation to effect through curing of the resin without need of using curing additives which would adversely affect the flexibility and adhesiveness of the resin.